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bisonyacc

match multiple patterns in bison

In lex/flex, one can match multiple patterns to a token using normal regex rules, but what is the equivalent on the yacc/bison side?

In my code I have two possible grammars for a single task:

IF expression THEN number
IF expression GOTO number

It seemed obvious...

|
IF expression THEN number
|
IF expression GOTO number
{ ... guts of the handler go here... }

But bison complains type clash on default action: <statement> != <>. Some googling turned up that if you don't have an action for a given entry it assumes the action is { $$ = $1; };, which is why you get this error. Fair enough.

It would seem this is a common sort of task, so I looked in a half dozen examples of little parsers on github and it seems it is not so common after all.

So what is the correct syntax here?

Solution

  • In yacc/bison syntax, each alternative has its own action, which is almost always what you want. There is no syntax for grouping several alternatives. So you would need to duplicate the action:

     if: IF expression THEN number { do_Something ($2, $4); }
   | IF expression GOTO number { do_Something ($2, $4); }

    If you don't like duplicate actions, group your semantically identical tokens:

     then: THEN | GOTO
 if:   IF expression then number { do_Something ($2, $4); }